X-ray detector for radiographic imaging

ABSTRACT

A method and apparatus for enhancing the optical readout efficiency of a storage phosphor medium used for x-ray detection is disclosed. A photoemissive cathode strip is utilized to convert optical photons emitted from the storage phosphor medium into photoelectrons. Additional photocathodes may be provided to emit photoelectrons from the photons which pass through the photoemissive cathode strip. Preferably a reflective backing on the additional photocathodes is provided to increase the collection efficiency. The photoelectrons are then directed to one or more output elements for collection and amplification. This invention is particularly useful in conjunction with a point or line scan format of readout. Optical detectors are constructed and arranged such that a substantial fraction of the photo signal emitted by the storage phosphor medium in converted into an electronic signal. Weighting of output signal may be done. In another embodiment, optical photons form two storage phosphor screens are detected by a semiconductor photosensor which is optically sensitive on both sides.

RELATED APPLICATION INFORMATION

This application is a continuation in part of application Ser. No.047,028 filed on May 5, 1987, now U.S. Pat. No. 4,937,453, entitledIMPROVED X-RAY DETECTOR FOR RADIOGRAPHIC IMAGING.

BACKGROUND OF THE INVENTION

Improvements on laser scanned storage phosphors systems have beendescribed by Nelson, in the above referenced copending application,principally by increasing the thickness of the phosphor screen or theuse of two screens to improve x-ray stopping power. In addition, methodsof weighting the relative importance of the output signals collectedduring scanning by the two laser beams were described.

Problems arise due to the poor optical coupling efficiency betweenphosphor and photomultiplier tube through the use of a long, lineararray of fiber optics. This may introduce additional noise into thesignal if the storage phosphor has poor conversion efficiency from x-rayenergy into an optical signal. In some cases a faster read out may bedesired, which is possible with a line-scan format but difficult toachieve with a point-scan format and a single photomultiplier tube.

SUMMARY OF THE INVENTION

Improvements to the readout unit employed with one or more storagephosphor or stimulable phosphor screens are provided which simplifydesign and/or improve optical detection efficiency of the light from thephoto-stimulable phosphor material. These improvements help maximize theprobability of detecting the x-ray signal, especially at low levels ofradiation intensity.

In one of the preferred embodiments, the storage phosphor screens areexposed in a normal manner. As is known, the storage phosphor screensare then scanned with laser light in order to stimulate emission of aphoton. In accordance with this invention, a readout unit for a storagephosphor screen is comprised of one or more photocathode strips, aregion where photoelectrons can be accelerated and focused, and anoutput element. The photocathode strips may be of a structured designand may preferably be backed by a reflective material to increase thedetection efficiency. The region where the photoelectrons areaccelerated and focused is maintained in a vacuum. The output element ispreferably a single element or linear array detector (which may provideadditional amplification). A single readout unit can then be used toscan a single screen or a dual readout unit can be used to scan twoscreens. The single screen or two screens having been previously exposedto x-rays. In the case of two screens, the screens can be separatedafter exposure for readout purposes. Several techniques can beimplemented to weight the optical signals from the two screens, ifdesired.

In another embodiment of the invention, readout of two screens can beaccomplished by a linear array of semiconductor photodetectors such asphotodiodes or CCD(s) or a double sided photodetector unit. Oneembodiment of the double sided detector is a back-illuminated CCD whichis optically sensitive from both front and back side, which can be usedto simultaneously readout two screens. This readout format avoids theneed for a vacuum container and can be compact. Optical filters ordifferences in quantum detection efficiency between the two sides of thephotodetector can be exploited to weight optical signals from the twoscreens. Likewise, the intensities of the scanning laser beams can beweighted.

Thus, a principal objective of the present invention is to provide animproved readout unit for use with x-ray storage phosphor screens inwhich a large fraction of the optical signal from the de-stimulatedstorage phosphor is converted into an output signal for analysis.

Another object of the present invention is to provide a readoutmechanism which allows two screens to be scanned simultaneously.

A further objective of this invention is to provide a means of weightingoptical signals from the two screens.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a strip readout unit comprised of avacuum chamber in which a long narrow photocathode strip is used toconvert the optical signal from the laser-scanned storage phosphorscreen into photoelectrons.

FIGS. 2a-2g show perspective views of several structured photocathodedesigns which enhance the probability of the optical signal from thestorage phosphor to interact with the photocathode material.

FIG. 3 shows a perspective view of a readout unit which permits twoscreens to be scanned at one time.

FIG. 4 shows a perspective view of a semiconductor photodetector withtwo optically active faces and appropriate light guides for two screenreadout.

FIG. 5 is a perspective view of a semiconductor photodetector with asingle active side.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention relates to a method and apparatus for detecting opticalradiation from a storage phosphor screen while the screen is scanned ina point or line scan format. In accordance with this invention, readoutdevices are constructed and arranged such that a substantial fraction ofthe optical signal from the screen is converted into an output signalsuitable for analysis by analog or digital means. In this way a readoutdevice is provided which promotes the conversion of optical intoelectrical signals while reducing the complexity of operation bylimiting the number of output elements to one or a linear array ofelements.

FIG. 1 shows a strip readout device for use in a point or line scandischarge of a storage phosphor screen of conventional or structureddesign, not shown. Optical photons 10 from the screen are converted bythe photocathode material 12 into electronic signals (photoelectrons)14. These photoelectrons 14 can be accelerated toward an output element16, resulting in a net gain in energy. Electron optics, not shown, maybe employed to map the photocathode onto the output element 16. Theoutput element 16 may be selected from a variety of conventional meansincluding:

1. electron amplification as is employed with photomultiplier tubes,

2. conversion by another phosphor into an optical signal which is thenrecorded by a photodetector,

3. bombardment of a CCD or other semiconductor device.

If minification is practiced, the output element 16 may have a muchsmaller area than its projection onto the photocathode 10. This promotesnoise reduction in the output signal. In addition, if a single outputelement is used rather than a linear array, construction costs aregreatly reduced although a point-scan format is then required.

Most of the assembly needs to be incorporated into a vacuum chamber 18.The photocathode strip 10 may be quite long, for example, up toapproximately 50 cm long, but it is typically expected to be ofrelatively narrow width, for example, less than several millimeters.This is because the width of a line of the image is typically much lessthan one millimeter, i.e., spatial resolution is often in the range offrom 1-8 line pairs per millimeter for medical imaging. Problems ofstructural integrity at the photocathode window are greatly reduced.

In comparison with the approach of using a fiber bundle to couple thephosphor screen signal to a photomultiplier tube, substantial benefit isderived by placing the photocathode material 12 very close to thephosphor screen to be read. The optical collection efficiency is muchhigher. Many storage phosphors are fairly weak scintillators incomparison to bright x-ray phosphors such as CsI:Na or _(G) d₂ O₂ S:Tb.Coupling with a long fiber bundle usually entails substantial opticallosses which can seriously degrade the optical photon statistics andintroducing additional noise into the output signal. (Reference: Nelson,R. et al., An Evaluation of a Fluorescent Screen-Isocon Camera Systemfor X-Ray Imaging in Radiology, Med. Phys. 9(5):777, 1982).

Readout is ordinarily accomplished by laser scanning an entire line ofthe screen at once or by point scanning with a laser beam across a line.Relative motion is introduced between the screen and readout device inorder to acquire an image.

Scanning a storage phosphor screen with a small readout devicerepresents a practical approach to permitting the screens to be freelyutilized throughout a radiology department or a factory setting. Afurther improvement in coupling efficiency between phosphor andphotocathode can be made by incorporating both into an intensifier unit,not shown, with a two dimensional receptor area. This approach reducesthe light losses encountered due to the small airgap between thephosphor screen and readout unit when they are separate units. Thephotocathode material may be deposited onto the phosphor medium or on anintermediate structural light guide. Typically, the laser or opticalbeam must scan from the same side of the phosphor material as x-rays areincident upon. A reflective coating could be applied to the scannedsurface if the laser beam is properly polarized and incident to thatsurface at the Brewster angle. An alternative is to create a window inthe vacuum housing to permit the laser beam to scan the phosphor screenfrom the vacuum side. The laser light must penetrate the photocathodematerial to discharge the phosphor. This dedicated unit benefits fromsimplified electron optics due to a single or linear array elementoutput rather than a 2-D array element output.

FIGS. 2 a-g shows several embodiments which implement a structuredphotocathode. As shown in representative FIG. 2a, photons 10 will beincident on a photocathode window 22. Photoelectrons 14 will be emittedand focused by electromagnetic fields, not shown. Photons which passthrough the photocathode window 22 may strike the back photocathode 24,which may also preferably contain a reflective material. Photoelectrons16 will be emitted and focused as for photoelectrons 14. A vacuum ismaintained in region 26.

Typical photocathode materials have a conversion efficiency less than20% and often under 10%. The photocathode material must be applied as avery thin film to ensure that a photoelectron has a reasonableprobability of escaping the material surface. Optical photons may have ahigh probability of being transmitted by the thin film photocathode 22.Introducing other photocathode structures, such as the back photocathode24, present additional opportunities for the photoelectric process tooccur.

Further improvement can be imparted to the structure by depositing ontothe appropriate surfaces a reflective coating prior to deposition ofphotocathode material. Such a reflective coating may be a white paint orteflon covering a low index material such as magnesium fluoride, etc. ora metallic reflector (Al, Ag, etc.) covering a low index material.Optionally, the low index material alone could be applied to create anindex of refraction mismatched at the photocathode material--low indexmaterial boundary. This creates an opportunity for an optical photon tomake a double pass through the photocathode material.

FIG. 2b shows a structure similar to FIG. 2a in that it has a frontphotocathode window 22 which serves to emit photoelectrons 14, and totransmit a portions of the photons 10 to the back photocathode andreflective surface 24. It will be appreciated that any structure whichobtains the benefits of this invention may be employed, as known bythose skilled in the art. The back photocathode is structured such thatthe walls between individual elements may be coated with photocathodematerial.

FIG. 2c shows an alternative embodiment comprising a plurality oftapered light guides 30. A tapered section 32 is coated withphotoemissive material. Photons 10 from a storage phosphor device passthrough a window 34 and are received by the various light guides 30.Photons 10 striking the photoemissive material 32 will causephotoelectrons 36 to be emitted. They may be focused and collected asdiscussed above in connection with FIG. 2a.

FIGS. 2d and e show two additional embodiments. The incident photons 10from the storage phosphor screen generate photoelectrons 14 in the firstpass, and other photoelectrons 16 from a second photocathode material24.

Two embodiments are shown using structured light guides in FIGS. 2f andg. Photons 10 from the storage phosphor screen, not shown, are incidenton a structured light guide 40. Photocathode material 42 may be used onthe surface of the light guide 40. A reflector 44 coated withphotoemissive material may be employed. In the case of FIG. 2g acontinuous structure 46 is provided to the rear wall of the detector. Byredirecting light to an internal photocathode, simplify the design ofthe vacuum--window interface and by providing a continuous structure tothe rear wall, improved performance results. It will be appreciated thatthere is no vacuum directly behind the entrance window.

The structure of the photocathode may influence the degree of cross talkbetween adjacent elements at the input. This can thus influence thechoice between a point or line scan readout format since spatialresolution would be degraded.

The units described in FIGS. 1 and 2a-g could be used to readout a pairof storage phosphorous screens. The output signals could be weighted andcombined or digitized and analyzed as in dual energy imaging.

As shown in FIG. 3, this invention may be advantageously used to scantwo screens simultaneously. Photons 10 from a storage phosphor screen13, are incident on two or more photocathode windows 52. Photoelectrons56 are emitted and focused by electromagnetic fields, not shown. Photonswhich pass through the photocathode windows 52 may be incident onphotocathode material and reflector 54. Photoelectrons 58 are emittedand focused as for photoelectrons 56. An output element 60 is providedto detect the photoelectrons 56, 58. A vacuum housing 62 is provided.

The advantage of such a structure is that the photoelectric signals fromcorresponding elements 52, 54 in the two screens can be combined at theoutput element 60 or output element array. Various means of weightingthe signals from the screens could be implemented:

(a) adjust the relative intensities of the scanning laser beams,

(b) introduce or incorporated a variable optical filter 11 between theback screen and the readout device,

(c) adjust the accelerating potential difference between the outputelement(s) and the two photocathode units, thereby provide unequal gain,or

(d) use separate output element(s) with adjustable amplification beforecombining the two signals.

Option d reduces simplicity in comparison to the use of a single outputelement or single array of output elements. Also, an increase in readoutnoise may occur. This approach is not much different from using twoseparate readout units and combining their outputs. This approachpermits dual energy analysis and is not limited to simple weighting ofsignals before combining.

FIGS. 4 and 5 show several means of scanning two screens using a readoutdevice based on semiconductor photodetectors. The readout device iscompact and does not require a vacuum structure. Filters can beintroduced to limit interactions of the scanning beam with thesemiconductor detector if needed. Filtration may also be added to weightsignals from the two screens. The intensity of the scanning beams couldalso be adjusted appropriately as a means of weighting signals from thescreens. Optical attenuators could be utilized if the scanning beamsoriginate from a common source. The use of two lasers permits electronicadjustment of beam intensity.

FIG. 4 employs a double sided semiconductor photodetector unit 72 suchas a back-illuminated CCD with a photosensitive front side or twophotodetectors with a common output. Appropriate light guides 70 such asfiber bundles are used to channel optical signals from the storagephosphor screen, not shown, to the photodetector. Ideally the distancebetween the screen and the photodetector is kept small so as to reducelight losses within the light guides.

FIG. 5 employs a single sided semiconductor photodetector 78 such as aphotodiode, CCD or amorphous materials such as amorphous silicon withappropriate light guides, such as curved or angled fiber bundles 74 ,along with a second light guide 76 so as to form a dual readout unit.

An alternative to the use of a long continuous photosensor surface, orthe abutment of photosensor units, is to create a continuous detector byabutment of the light guides. Nearest neighbor photosensor units arepositioned on opposite sides of the continuous light guide structure.This format makes for easy replacement of a defective photosensormodule.

What is claimed is:
 1. A detector for optical photons from a storagephosphor medium comprising,a first photocathode, said first photocathodecomprising means for receiving optical photons emitted from a storagephosphor medium, and a second photocathode arranged to receive photonspassing through the first photocathode.
 2. The detector of claim 1wherein the second photocathode further comprises a reflective backing.3. The detector of claim 1 further comprising a detector for sensing thephotoelectrons emitted from the first and second photocathodes.
 4. Thedetector of claim 1 further comprising a vacuum chamber.
 5. An apparatusfor the detection of optical photons emitted from two storage phosphorscreens comprising:two strip photocathodes oriented relative to theirrespective storage phosphor screens, a single vacuum container withfocusing electron optics and output element(s) common to bothphotocathode strips.
 6. An apparatus for the detection of opticalphotons emitted from two storage phosphor screens upon irradiation byone or more scanning beams comprising:a semiconductor photosensor meansfor conversion of optical photons into an electrical signal, light guidemeans for transmitting the optical signals from the phosphor screens tothe photosensor means and optical filter means for weighting therelative importance of signals from the screens.
 7. The apparatus ofclaim 6 further comprising filter means for minimizing transmission of ascanning beam to the photosensor means.
 8. The apparatus of claim 6further comprising optical attenuator means to control one or moreintensity of scanning beams.
 9. An apparatus for the detection ofoptical photons emitted from two storage phosphor screens comprising:asemiconductor photosensor unit which is optically sensitive on bothsides and in which the two sides share a common collection andamplification means and light guides comprising means for transmittingthe optical signals from the phosphor screens to the optically activefaces of the photosensor unit.
 10. An apparatus as described in claim 9in which the semiconductor photosensor unit is a double-sided detector.11. An apparatus as described in claim 10 in which the double-sideddetector is a back-illuminated CCD.
 12. A detector for optical photonsemitted from a storage phosphor medium employed for radiographic imagingcomprising:a plurality of individual strip photocathode means structuredfor enhancement of conversion probability from optical photons emittedfrom a storage phosphor medium into photoelectrons, electron optic meansfor focusing the photoelectrons, and a vacuum container in which thephotoelectrons are focused, and a detector for collecting at least aportion of the output of the photocathode means.